Rapid heating systems, also called RTP systems, RTP installations or rapid heating installations, are widely known in the production of semiconductors, and are described, for example, in U.S. Pat. No. 5,359,693 or U.S. Pat. No. 5,580,830. They are used for the thermal treatment of substrates, in particular disc shaped substrates, such as for example semiconductor wafers. Rapid heating systems are a fixed component part of many semiconductor production lines. In order to obtain the desired process results they must heat a wafer to be treated according to a pre-specified temperature/time curve, keep the semiconductor substrate at a specific temperature for a pre-specified period of time, and finally cool the substrate down again, also mostly according to a pre-specified temperature/time curve. During these processes it is particularly significant that the temperature of the semiconductor wafer corresponds as precisely as possible to the respectively pre-specified temperature, and this temperature is as homogenous as possible over the wafer. As well as the requirements with regard to temperature precision, the dynamics of the heating processes are also very important because meanwhile, many industrial semiconductor production methods require rapid heating and cooling cycles. The active elements of a rapid heating system must therefore have sufficiently short reaction times. For this reason, with rapid heating systems lamps (halogen lamps or arc lamps) or also lasers are used in most cases as heating elements because on the one hand they can be controlled precisely, and on the other hand allow very rapid heating cycles.
As a consequence of the rapid thermal processes, a temperature measurement in the processing chambers of rapid heating systems must also be implemented very quickly during a thermal process in order to guarantee that pre-specified temperatures are maintained, and if necessary to adapt the control of the heating elements. Furthermore, it is important here that the temperature measurement does not have a negative impact upon the thermal treatment of the wafer itself. Therefore, in many cases only contact-free, in particular pyrometric temperature measurement systems are considered. Thermal elements react too sluggishly and so are mainly used for calibration processes for pyrometric temperature measurement systems.
Contact-free temperature measurement systems establish the temperature for example by measuring radiation emitted from the object (self radiation), the temperature of which is to be determined, directly or indirectly. With applications with which high temperatures must be measured, such as for example in rapid heating systems, pyrometers are mainly used. Considered as detectors for pyrometers which are used in this field are, for example, bolometers, pyroelectrical sensors, temperature-dependent resistors (e.g. thermopiles) or photoelectrical detectors. Pyrometers convert the detected radiation directly or indirectly into an electrical measuring signal, and in most cases have the further advantage that they can react sufficiently quickly to changes in the thermal radiation emitted by the substrate to be treated during the thermal process. Depending on the area of application, pyrometers can be used in environments with very low and also with very high radiation density, and so determine the temperature of objects in ranges of between one Kelvin and several thousand Kelvin.
When using pyrometers in rapid heating systems, in general the pyrometer not only receives the self radiation of the substrate to be treated thermally at which it is directed however, but also direct and indirect (reflected and multiply reflected) radiation portions from the substrate environment which are emitted, for example, by the elements heating the substrate (lamps, lasers, resistor heating elements etc.) and the radiation from adjacent objects. It is therefore a challenge to extract the portion of the thermal radiation actually emitted by the substrate from the overall signal hitting the pyrometer in order to determine the substrate temperature from this portion.
Numerous methods for contact-free determination of the temperature of substrates, such as for example wafers, during a thermal process are known from the literature. Although many of these methods are predominantly used for determining the temperature of a semiconductor wafer in a rapid heating installation, they can also be transferred to determining the temperature of objects in ovens.
A method for contact-free temperature determination is disclosed, for example, in the patents of the Applicant U.S. Pat. No. 6,191,392 and U.S. Pat. No. 6,396,363. With this contact-free temperature measurement a first pyrometer is provided which is directed onto one side of the wafer in order to collect the radiation coming from the wafer. This radiation contains both the heat radiation of the wafer itself, and also the radiation from the radiation sources reflected on the wafer. If the wafer is transparent or at least semi-transparent for the selected radiation range, it can also contain portions of the radiation from the radiation sources measured during transmission through the wafer. Furthermore, a second pyrometer is provided which is directed directly towards the radiation sources in order to collect the radiation emitted from the radiation sources. In order to be able to distinguish the self radiation of the wafer from the direct or reflected radiation, or the radiation passing through the wafer, the lamp radiation is modulated. This makes it possible for the radiation source portion which the first pyrometer detects, for example, due to a reflection on the wafer, to be determined and to be subtracted from the overall radiation detected.
The modulation of the radiation sources is chosen here such that it can be measured in the radiation from the radiation sources reflected on the wafer or passing through the wafer, but is not reflected in a modulation of the self radiation of the wafer due to the thermal inactivity of the wafer. With this method, by means of a particular algorithm, the lamp radiation measured on the second pyrometer can be determined and be subtracted from the overall radiation measured on the first pyrometer so that in this way the self radiation of the wafer can substantially be established from the overall signal. From the remaining self radiation, the temperature of the wafer can then be determined. The above method is also known as the ripple technique.
The advantage of the ripple technique is that it is substantially independent of the optical properties of the wafer because it determines the self radiation of the wafer independently of the optical properties of the latter. With very dynamic heating processes the technique touches on its limits, however, and in general short-term deviations between the actual temperature of a wafer and the temperature determined can occur. These deviations can in turn lead to false control of the heating elements which is implemented upon the basis of the determined temperature. With sudden changes to the optical properties of the wafer, as occur for example with recrystallisation process, this sudden change can also have a negative impact upon correct temperature determination in the short term, and this in turn can lead to false control of the heating elements.
With an alternative method measurement of the lamp radiation by means of a second pyrometer, as described above, is replaced by a model which, for example, calculates the resulting modulated lamp radiation upon the basis of the respective control performance of the heating elements. If for example one has very good knowledge of the properties of the processing chamber and of the heating elements and the properties of the wafer to be heated, a mathematical model for in-situ determination of the temperature of the wafer to be heated can be developed. With a sufficiently accurate model of the substrate and its environment, it is possible to calculate lamp background radiation by means of, for example, the control parameters of the heating elements in order to thus determine the substrate temperature directly from the detected heat radiation (i.e. from the sum of the detected self radiation of the wafer and the detected lamp background radiation). The lamp background radiation is to be understood here as meaning all of the radiation detected by the radiation detector less the self radiation of the object of which the temperature is to be determined.
The Applicant's WO 2004/059271 discloses a model-based method (model method) for determining at least one state variable (for example the temperature of a semiconductor wafer) from a model of an RTP system by means of at least one measuring signal picked up on an RTP system which has a dependency upon the state variable to be determined and by means of a corresponding prediction value of the model.
The quality of the model predictions depends upon how precisely they are described by reality. Therefore, all parameter values and boundary conditions having an effect upon the model should be sufficiently precisely known or it should at least be possible to determine them sufficiently precisely. Therefore, models for rapid heating systems include at least the properties of the substrate to be heated (such as e.g. its physical parameters), so that they are capable of producing “response functions” to thermal stimulations in a way that is to a certain extent true to life. On the other hand however, chamber and lamp properties, such as for example chamber reflectivity, lamp radiation, lamp reaction times, the lamp response to electrical impulses, effects of the lamp background radiation etc. have an effect upon the thermal stimulation of the substrate to be heated. These determine the thermal energy actually having an effect upon the wafer and the energy which the wafer can release as self radiation back to its environment per unit of time. These values can only be influenced by the energy which can be introduced to and be discharged from the overall system per unit of time. Since, however, these parameters co-determine the response function of the substrate to be heated, a complete model must also record these parameters i.e. contain at least one model of the substrate environment.
For reasons relating to the real-time requirement, but also because not all required parameters can be determined sufficiently precisely, in general complete model descriptions can not be produced, and this is why the system models used are always reduced models which are limited to a reduced number of more or less precisely determinable state variables. Often, it is also only possible to model part systems and the missing aspects must in many cases be made up for by a corresponding number of partially intricate measurements, and if appropriate be corrected, mostly not all of the required values of the direct measurement being accessible. Therefore, a sufficiently precise measurement of the lamp background radiation having an effect upon the object to be heated for example is very complex because additional measuring means, for example further pyrometers, are required in order to determine this precisely in order to gain precise knowledge of, among other things, the properties of the rear side of the wafer. In the previously used model systems the lamp background radiation is therefore calculated.
If one presumes that the initial state of a real system and the initial state of a system model representing this system are exactly the same, and if one further presumes that the system model exactly replicates the dynamic characteristics of the real system and that no noises or disturbances have any effect upon the real system, the states of the real system and of the system model illustrating this system will always develop in the same way with the same control values. Since, however, the system model only contains partial aspects of the overall system, it must be completed by a control which, if appropriate, takes into account any disturbances that occur. Therefore, with the aforementioned WO 2004/059271 initial variables (measurements) for a closed loop control system (real system) are compared with those of a system model (observer), and differences between them retroact by means of a regulator upon the state of the observer. By means of this state correction, the state of the system model is adapted to the real system, or in other words, the adjustment value for adaptation of the system model is established by an algorithm which compares a recorded measurement value for an object in the rapid heating installation with a prediction value for the measurement value of the object, and is intended to minimise the difference between these two values. In most cases this method makes it possible to determine the thermal characteristics of a wafer very precisely during its thermal treatment.
If, however, the disturbances show unexpected dynamic characteristics which overextend the algorithms typically used, errors can arise due to model misinterpretations. Such cases can occur in particular with very dynamic rapid heating processes and/or if the wafers have a very high level of reflectivity. They can also occur, for example, if an optical property, such as for example reflectivity, suddenly changes during the thermal treatment of the wafer, as was observed e.g. with recrystallisation processes during a thermal treatment. A precise model prediction is of course made particularly difficult if these processes occur in combination, if for example a very dynamic rapid heating process takes place with a wafer with a high level of reflectivity. One reason for this is that these processes are generally not taken into account in system models because they occur sporadically, and also not with all processed wafers. A further reason is a time delay which comes about because the currently measured signals which generally have a lot of noise, are smoothed in filters for the purpose of high signal precision in order to average statistical fluctuations which occur between measurements following one another directly over time. The value determined for the lamp background radiation (lamp radiation coming directly and indirectly into the pyrometer), which is not measured independently of the substrate radiation, can therefore be associated, at least in the short term, with an error relevant to the temperature measurement. This can lead to erroneous temperature determination because, for example, parts at least of the lamp background radiation are assigned to the self radiation of the wafer. The result of this is that the parameter values of the model are; wrongly corrected, and this in turn leads to faulty reactions of the overall system, such as for example an oscillation of the radiation sources because the overall system now tries to readjust the temperature of the wafer determined in this way. Subsequent measuring errors caused by this can lead to erroneous temperature determination over many individual measuring steps, and so lead to instability of the whole system control.
Moreover, the above system is problematic with highly reflective (for example metal-coated) wafers. With these wafers a change to the lamp background radiation dominates very strongly in relation to a change to the self radiation of the wafer. Small errors when determining the lamp background radiation lead to large errors when determining the self radiation (and so the temperature) of the wafer because the differential signal from the overall radiation and the subtracted lamp background radiation is very small. An error which may possibly already be present in the wafer temperature can lead to further destabilisation of the adjustment algorithm upon the basis of high lamp dynamics.
If with a highly reflective wafer the reflectivity changes abruptly during a thermal treatment cycle, the resulting pyrometric measuring signal can be considerably misinterpreted by the evaluation unit (erroneous changes in the reflectivity measurement values are interpreted as a change to the wafer emissivity), and an unrealistic temperature is determined. These abrupt changes can be caused, for example, by phase transformations in applied layers, evaporation or also alloy formations, and are generally not taken into account in model systems because these results can be different from wafer to wafer depending on the coating of the rear side of the wafer, and therefore do not occur predictably. Although the system generally recognises the change in reflectivity over time and correspondingly incorporates this into the temperature determination, it can meanwhile lead to unstable operation of the system with substantial fluctuations as regards the lamp radiation.
If thermal processing steps are provided in a temperature range in which the semiconductor substrate to be processed is almost transparent for thermal radiation, the same problem occurs as with a highly reflective wafer: If the portion of lamp background radiation transmitted by the wafer compared with the thermal radiation of the wafer is very high, small changes to the lamp radiation lead to large relative changes when determining the thermal self radiation of the wafer if the measuring system detects the sum of the very small self radiation of the wafer and the transmission signal of the lamp radiation. Relative measuring errors when establishing the overall signal increase the relative error when determining the self radiation of the wafer which is formed by the difference formed from two values of approximately the same size, namely from the overall signal and the lamp background signal. In this case too precise direct pyrometric measurement of the self radiation of the wafer is made difficult.
Therefore, the object which forms the basis of the present invention is to provide a method and an apparatus for determining measurement values of a measuring system which relate to at least one parameter of an object in a rapid heating apparatus which guarantees reliable determination of the measurement values independently of dynamic process management or sudden changes within the rapid heating installation. In particular, precise temperature determination of an object in the rapid heating apparatus is to be guaranteed, even with highly dynamic processes and with abrupt changes to the optical properties of the object.
In particular the present invention provides a method for determining measurement values which relate to at least one parameter of an object in a rapid heating system. With the method the measurement values are determined by means of at least one measurement measured over time in a measuring system and prediction values for the measurement values in at least one model system are calculated. Furthermore, the method has the following steps: calculating a measurement value by means of the at least one measurement which was recorded at a first point in time, calculating a first prediction value for the measurement value at the first point in time, calculating a second measurement value by means of the at least one measurement which was recorded at a second point in time, calculating a second prediction value for the measurement value at the second point in time, comparing the development over time between the first and second measurement value with the development over time between the first and second prediction value, establishing a corrected second measurement value if the development over time between the first and second measurement value differs from the development over time between the first and second prediction value, and issuing the corrected second measurement value from the measuring system.
By means of the above method, a development over time between actual measurement values and prediction values for these measurement values can be compared by means of which errors, which occur within the measuring system, can be identified and corrected. Even if the prediction values of the model system can not precisely indicate the (absolute) measurement values, they can indicate sufficiently precisely how the measurement values develop over time. Also, should the situation arise where the absolute values differ more greatly, a difference in the respective development over time between the measured and prediction values would point to an “error” in the measuring system which it is appropriate to correct. In relation to the thermal treatment of semiconductor wafers, a sudden change in the reflectivity of the wafer could for example first of all result in incorrect calculation of the wafer temperature because the measuring system relates this change in reflectivity and, if applicable, the associated greater amount of radiation to which the pyrometer is subjected to, to a corresponding abrupt temperature change. Only after a certain time would the measuring system recognise that the sudden change in the amount of radiation to which the pyrometer is subjected to, is not dependent upon a corresponding temperature change, and would, for example, undertake a corresponding correction by means of the ripple technique. In order, however, to avoid an incorrect temperature reading meanwhile, which can in turn influence the control characteristics of the overall system, the method described above can correct excessively great changes to the measurement values which are caused by a false interpretation of their cause by means of the prediction values of the model system.
According to one particularly preferred embodiment of the invention, the corrected second measurement value is only established if the difference in the development over time between the first and second measurement value and the development over time between the first and second prediction value exceeds a pre-determined value so as to only undertake a correction if there are substantial differences. Negligible deviations could rather indicate that the model system can not totally precisely replicate the circumstances within the rapid heating system.
Preferably, the measuring system uses as a measurement an output signal from a radiation detector which records temperature radiation from an object located within the rapid heating system because the above system is suitable in particular for temperature radiation measurements and so for related parameters. Here, the calculated measurement value is preferably a value for the temperature, radiation, emissivity, transmissivity and/or the reflectivity of the object. For example, the measuring system can calculate the measurement value independently of the emissivity of the object. An example of measurement value determination independent of the emissivity is for example temperature determination by means of the “ripple technique” described above or a model-based technique, as described in WO 2004/059271 A, which in this respect is incorporated herein by reference.
Preferably, the temperature radiation from the object located within the rapid heating system is recorded during the thermal heating process of a semiconductor wafer located within the rapid heating system because the above system is particularly suitable for controlling the temperature of a semiconductor wafer in a rapid heating system.
With one embodiment of the invention, in order to correct the second measurement value, the latter is replaced by the second prediction value. With an alternative embodiment a corrected second measurement value is calculated by means of the first measurement value and a relationship between the first and second prediction values. With a further alternative embodiment, the corrected second measurement value is calculated by means of the first and second measurement values and the first and second prediction values. Here, the second measurement value is preferably calculated by means of the formula
            f              t        ⁢                                  ⁢        2            *        =                  f                  t          ⁢                                          ⁢          1                    +                        f                      t            ⁢                                                  ⁢            2                          ×                  (                      1            -                                          g                                  t                  ⁢                                                                          ⁢                  1                                                            g                                  t                  ⁢                                                                          ⁢                  2                                                              )                      ,f*t2 representing the corrected measurement, ft1 the first measurement value, ft2 the second measurement value, gt1 the first prediction value, and gt2 the second prediction value.
In order to provide corresponding correction over the process, further measurement values and prediction values can be repeatedly calculated, it being possible for a previously corrected measurement value to form the basis for a subsequent comparison in so far as a correction is implemented.
According to one particularly preferred embodiment of the invention, the model system has at least one model of a semiconductor wafer and/or a model of a processing chamber of the rapid heating system and/or at least one model value for the reflectivity of the object. Here, the model system preferably calculates the value of the background radiation surrounding the object. Furthermore, the model system preferably has at least a first and/or a second changeable model value so as to be able to adapt the model system, if appropriate, during a process. Here, the first changeable model value is preferably changed if the absolute value of the difference between the second measurement value of the measuring system and the second prediction value of the model system is smaller than a pre-specified value. On the other hand, the second changeable model value is preferably changed if the absolute value of the difference between the second measurement of the measuring system and the second prediction value of the model system is greater than or equal to a pre-specified value. Here, it is preferably the second changeable model value which influences the model value for the reflectivity of the object. If the reflectivity of the object changes abruptly, this should also be taken into account within the model system, and this is possible by means of a corresponding model value change. Alternatively and/or in addition, the second changeable model value could influence a model value for the background radiation surrounding the object, by means of which in turn adaptation of the model system to, for example, a change in reflectivity of the object is possible.
With one preferred embodiment of the invention the model system calculates the prediction value for the temperature radiation recorded by the detector. Here the first changeable model value preferably depends functionally upon the prediction value of the temperature radiation calculated for the measuring system. With small deviations therefore, by means of a corresponding change to the first changeable model value adaptation of the model system to actual measurements can be implemented.
The object forming the basis of the invention is also fulfilled by a method for determining the temperature of an object in a rapid heating system with at least one measuring system which records as the measurement at least one temperature radiation from an object located in the rapid heating system and determines a temperature measurement value by means of the measurement, the temperature measurement being selectively correctable by means of a correction method as described above.
The object specified above is fulfilled furthermore by an apparatus for establishing at least one measurement value which relates to a parameter of an object in a rapid heating system, the apparatus having at least one measuring system for recording measurements of the object following on from one another over time and for determining measurement values from the measurements, at least one model system of the rapid heating system which calculates prediction values for the measurement value dependently upon time, means for comparing development over time of the measurement values with the development over time of the prediction values, and means for correcting the measurement values if the development over time between two measurement values differs from the development over time between two prediction values. This type of apparatus makes it possible to carry out the aforementioned method with the corresponding advantages.
In the following, the present invention is described in greater detail by means of preferred embodiments with reference to the drawings. The drawings show as follows: